At first glance, a planet looks just like a star -- a pinpoint of light in the night sky. But if you look carefully, there are several ways to tell them apart.

Consider the planet Mars and Regulus, the brightest star of Leo. They’re high in the southeast as night falls this month. And tonight, they line up to the lower left of the Moon, with Regulus closer to the Moon.

Mars is the brighter of the two, and shines orange to Regulus’s white. But that alone isn’t enough to tell us just what they are -- there’s a big variation in the brightness of visible stars, and several of them also look orange.

If you watch carefully, though, you’ll see that Regulus twinkles much more than Mars does. Regulus is so far away that it really is just a pinpoint of light, while Mars forms a tiny disk, so its light doesn’t appear as distorted by its trip through the atmosphere.

If you watch the two over many nights, though, you’ll really be able to see a difference between the star and planet.

Because Mars is quite close by, for example, it moves against the background of stars. It’ll stay close to Regulus for a while, but by the summer it’ll be moving away from it at a good clip.

And for the most part, a star’s brightness doesn’t change. But a planet can vary dramatically. A month from now, Mars will be just half as bright as it is tonight. And by late summer, it’ll be just one-sixth as bright. But it’ll brighten again next year -- part of the fickle nature of a planet.

Script by Damond Benningfield, Copyright 2012

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